1. Field of the Invention
The present invention relates in general to electron beam sources and more particularly to a field emitter array-type electron beam source.
2. Description of the Prior Art
The generation of macroscopic electron beam currents through vacuum field emission from a large number of emission sites requires a surface with a complex microstructure. To date, the fabrication of surfaces suitable to this task has been dominated by microlithographic techniques. In these processes, masks are used in conjunction with etching or deposition techniques to produce arrays of micron-scale cones or wedges.
The limitations of present electron source technology are experienced on a regular basis by those involved in microwave devices, high energy particle accelerators, laser pumping, and a host of other fields which utilize electron beams as a means of energy transfer. Presently available electron beam sources divide into three categories: thermionic emitters, laser-activated photoemitters, and field emitters. Included in this last category are both exploding field emitters, sometimes termed plasma cathodes, and vacuum field emitters which do not form an intermediate plasma. Recent interest in vacuum field emission has concentrated on microlithographically fabricated field emitter arrays that employ a nearby gate electrode.
Laser-activated photo-emitters use a high-power, short pulse laser to photo-eject electrons from a Cesiated cathode surface. Current densities greater than 400 A/cm.sup.2 at the cathode surface have been reported, for short pulses. The short pulse nature of this type of cathode is dictated by the high-power laser necessary to activate the cathode surface. The vacuum requirements of 10.sup.-10 torr or better place this cathode in the ultra-high vacuum range, making it infeasible for widespread use.
Thermionic cathodes presently available commercially have a cathode current density of no more than 20 A/cm.sup.2, with a required vacuum pressure of between 10.sup.-7 and 10.sup.-8 Torr during operation. There are research thermionic cathodes, based on scandate surfaces, which might generate current densities on the order of 100 A/cm.sup.2. These cathodes suffer from short lifetime and non-uniform emission. Equally as important is the list of materials not available for intra-vacuum use because they will poison the sensitive, Barium-Oxide or Lanthanum-hexa-Boride emission surface: nickel, gold, Steatite (non-Al.sub.2 O.sub.3 ceramics), iron (steels), platinum, titanium, carbon, tantalum, hydrocarbons, carbon dioxide, sulfur hexafluoride, and others.
Field emission cathodes, and particularly the explosive type, are the simplest class of cathodes to use, but, in another sense, are the most limited. Field emission cathodes operate by applying a large electric field to an emission surface, perhaps reactor grade graphite (carbon). The large field draws electrons out of the material by quantum tunneling. Presently, this process describes only the initial phase of "turn on". The initial current generated in this phase is emitted from small microscopic protrusions in the surface of the material; the large currents drawn though these small tips results in large local Ohmic heating of the tips, which subsequently ablate and produce a cathode surface plasma. Subsequent emission of electrons occurs from this cathode plasma, which has a very low work function and allows for high current densities to be generated (I&gt;100,000 A/cm.sup.2). The significant drawback of this process is that the generated cathode plasma typically expands towards the anode at a rate of 1 to 2 cm/.mu.sec, which limits the useful pulse length and precludes repetitively pulsed operation (the expanding plasma reduces the effective cathode-to-anode distance because emission occurs from the leading edge of the plasma. The decreased cathode-anode spacing increases the current which is drawn, since this type of situation is described by Child-Langmuir space-charge limited flow, resulting in electron gun impedance collapse). Field emission cathodes which operate without producing a plasma have to date been limited to single-tip or few tip emission arrays, which are not useful for the generation of macroscopic electron beams.